1. Field of the Invention
The present invention relates to a communication system for transmission/reception of a digital signal through modulation of its carrier wave and demodulation of the modulated signal.
2. Description of the Prior Art
Digital signal communication systems have been used in various fields. Particularly, digital video signal transmission techniques have been improved remarkably.
Among them is a digital TV signal transmission method. So far, such digital TV signal transmission system are in particular use for e.g. transmission between TV stations. They will soon be utilized for terrestrial and/or satellite broadcast service in every country of the world.
The TV broadcast systems including HDTV, PCM music, FAX, and other information service are now demanded to increase desired data in quantity and quality for satisfying millions of sophisticated viewers. In particular, the data has to be increased in a given bandwidth of frequency allocated for TV broadcast service. The data to be transmitted is always abundant and provided as much as handled with up-to-date techniques of the time. It is ideal to modify or change the existing signal transmission system corresponding to an increase in the data amount with time.
However, the TV broadcast service is a public business and cannot go further without considering the interests and benefits of viewers. It is essential to have any new service appreciable with existing TV receivers and displays. More particularly, the compatibility of a system is much desired for providing both old and new services simultaneously or one new service which can be intercepted by either of the existing and advanced receivers.
It is understood that any new digital TV broadcast system to be introduced has to be arranged for data extension in order to respond to future demands and technological advantages and also, for compatible action to allow the existing receivers to receive transmissions.
The expansion capability and compatible performance of the prior art digital TV system will be explained.
A digital satellite TV system is known in which NTSC TV signals compressed to an about 6 Mbps are multiplexed by time division modulation of 4 PSK and transmitted on 4 to 20 channels while HDTV signals are carried on a single channel. Another digital HDTV system is provided in which HDTV video data compressed to as small as 15 Mbps are transmitted on a 16 or 32 QAM signal through ground stations.
Such a known satellite system permits HDTV signals to be carried on one channel by a conventional manner, thus occupying a band of frequencies equivalent to the same channels of NTSC signals. This causes the corresponding NTSC channels to be unavailable during the transmission of the HDTV signal. Also, the compatibility between NTSC and HDTV receivers or displays is hardly concerned and data expansion capability needed for matching a future advanced mode is utterly disregarded.
Such a common terrestrial HDTV system offers and HDTV service on conventional 16 or 32 QAM signals without any modification. In any analogue TV broadcast service, there are developed a lot of signal attenuating or shadow regions within its service area due to structural obstacles, geographical inconveniences, or signal interference from a neighbor station. When the TV signal is an analogue form, it can be intercepted more or less at such signal attenuating regions although its reproduced picture is low in quality. If the TV signal is a digital form, it can rarely be reproduced at an acceptable level within the regions. This disadvantage is critically hostile to the development of any digital TV system.
It is an object of the present invention, for solving the foregoing disadvantages, to provide a communication system arranged for compatible use for both the existing NTSC and introducing HDTV broadcast services, particularly via satellite and also, for minimizing signal attenuating or shadow regions of its service area on the grounds ground.
The present invention provides a communication system comprising: input means for inputting an image signal; image compression means for compressing said image signal to generate a digital image compression signal; error correction encoding means for adding an error correction code to said digital image compression signal to generate an error correction coding signal; modulating means for modulating said error correction coding signal into an n-value VSB modulation signal; transmitting means for transmission transmitting said n-value VSB modulation signal; receiving means for receiving a transmission signal transmitted from said transmitting means; demodulating means for demodulating said transmission signal into a receiving digital signal; error correction means for error correcting said receiving digital signal to generate an error-corrected digital signal; image expansion means for expanding said error-corrected digital signal to generate an image output signal; and output means for outputting said image output signal.
It is preferable that the n-value of the VSB modulation signal is 8. Furthermore, a Trellis decoder will be used for the error correction means.
Moreover, the receiving digital signal is divided into a high priority signal and a low priority signal, and the error correction means includes a first error correction means with a high code gain and a second error correction means with a low code gain, so that the high priority signal is corrected by the first error correction means.
The high priority signal contains address information of data. And, the Trellis decoder will be preferable as the first error correction means.